Second iteration of the RF door lock. Old version worked but drew too much quiescent current. Sensor and servo pulled 13.8mA and 4.6mA idle. Linear regulators were a disaster. Battery didn’t last 24 hours.

Redesigned the PCB completely. Tossed the RF modules and the second MCU. Connected R503 sensor directly to the ATmega328P. Sensor now mounts on door exterior, servo attaches to interior knob, MCU stays on the back to prevent tampering.

PCB specs: 2-layer, 1oz copper, 0.3mm traces (1mm for power and servo). Ground plane on bottom layer.

Solved idle power draw with MOSFETs. 2N7000 and NDP6020P cut power to sensor and servo before deep sleep. Through-hole MOSFETs that switch at 3.3V are getting harder to find. NDP6020P already obsolete.

Replaced linear regulators with MP1584EN DC-DC buck converters. No RF, noise isn’t a big concern. Buck’s pin breakout not great–wouldn’t fit most commercial dev boards.

Squeezed more power savings from the MCU by running it at 3.3V/8MHz. Combined with buck converters: 56% total power savings.

Wake sequence: MCU activates sensor MOSFET, unlocks sensor over UART, scans and matches fingerprint. Match triggers blue LED, servo MOSFET, PWM signal to unlock. No match triggers red LED. MOSFETs off, back to sleep.

Total power savings: 99.9% (30.6mA → 2.9μA). Verdict: Fixed.

Commit: 7529094 | Gerber: gerber.zip